U.S. Pat. No. 5,304,208, incorporated herein by reference, describes a pacemaker of the rate-responsive type which includes an accelerometric sensor located in the pacing lead with the aim of making the pacing rate of the device depend on a parameter obtained by processing the signal of the Natural Heart Acceleration (NHA). U.S. Pat. No. 5,304,208 discloses the analysis of the entire signal which describes the NHA, and in particular the identification of the peak value, the calculation of the mean, and the possible identification of the atrial and ventricular phases; contemplating the utilization of the NHA signal for both single-chamber and dual-chamber cardiac pacemakers, for antiarrhythmic pacing devices and for implantable defibrillators, i.e., the so-called Pacer Cardioverter Defibrillators (PCDs). U.S. Pat. No. 5,304,208 differentiates the principles of measuring acceleration from those of measuring the blood pressure inside the ventricle and its derivative with respect to time. The same considerations can be extended to U.S. Pat. No. 4,936,304, where a cardiac pacemaker of the rate-responsive type is assumed, in which the pacing rate is regulated by the processing of intramyocardial stresses measured by inserting inside the myocardium a pressure meter of the type conceptually similar to the one used for measuring the ventricular blood pressure. Consequently, well documented from U.S. Pat. No. 5,304,208 is the substantial difference existing between the measurement of acceleration, where the accelerometric sensor is contained in an undeformable capsule, and the pressure transducers that are equipped with a deformable membrane which enables the pressure to be transduced into a measurable force.
The limitations in the use of the NHA signal according to the procedure described in U.S. Pat. No. 5,304,208 are based on the fact that, since the cardiac mass is subject to overall movements deriving from inertial reactions which are difficult to foresee and are due to the simultaneous phases of aortic and pulmonary ejection, and in association with these the movements themselves of the specific segment of endocardium in which the lead is inserted with the accelerometer, it may prove difficult from the mere reading of the accelerometric signal of NHA as a whole, to deduce which part is due to the overall movement and which expresses the muscular dynamics of the myocardium underlying the surface of the endocardium on which the accelerometer is set.